In an optical communication system put into practical use up to now, a two-level modulation and demodulation technique using light intensity is utilized. Specifically, “0” and “1” of digital information are converted into on-off of intensity of a light at a transmitter side, and transmitted into an optical fiber. The light that has propagated through the optical fiber is subjected to photoelectric conversion at a receiver side to restore original information. In recent years, with explosive growth of the Internet, a communication capacity required for the optical communication system develops dramatically. Up to now, an on-off speed of the light, that is, a modulation speed has increased in response to a request to make the communication capacity be huge. However, a technique in which the modulation speed increases to realize the huge capacity generally suffers from the following problems.
There arises such a problem that a transmittable distance limited by chromatic dispersion of the optical fiber becomes shorter as the modulation speed increases. In general, the transmission distance limited by the chromatic dispersion becomes shorter as the square of a bit rate. That is, when the bit rate doubles, the transmission distance limited by the chromatic dispersion is reduced to ¼. Likewise, there arises such a problem, that when the modulation speed increases, a transmittable distance limited by polarization mode dispersion of the optical fiber becomes shorter. In general, when the bit rate doubles, the transmission distance limited by the polarization mode dispersion is reduced to ½. Influences of the chromatic dispersion will be described specifically. If the bit rate is 10 Gbps, and a normal dispersion fiber is used, the transmission distance limited by the chromatic dispersion is 60 km. However, in a system having the bit rate of 40 Gbps, the transmission distance is shortened to about 4 km. Further, in a next-generation 100 Gbps system, the transmission distance limited by the chromatic dispersion is 0.6 km, and a trunk optical communication system having the transmission distance of about 500 km cannot be realized without any improvement. In order to realize an ultrahigh-high speed trunk communication system, a specific optical fiber such as so-called “dispersion compensating fiber” having a negative chromatic dispersion for cancelling the chromatic dispersion of a transmission channel is currently installed in a repeater or a transceiver. This specific fiber is expensive, and an advanced design such as how long the dispersion compensating fiber is installed at each site (a length of the dispersion compensating fiber) is required. Those factors drive up the price of the optical communication system.
Under the circumstances, in recent years, as an optical modem system that increases the communication capacity, study of the optical communication system using the OFDM technique enters the limelight. In the OFDM technique, respective amplitudes and phases of a large number of sinusoidal waves (called “subcarriers”) orthogonal to each other within one symbol time, that is, each having a frequency of the integral multiple of an inverse in one symbol time, are set to given values to carry information (modulate), and a carrier is modulated with a signal bundling those subcarriers and transmitted. The OFDM technique is used in a very high bit rate digital subscriber line (VDSL) system that communicates between a telephone exchange and a home, a power line communication system in the home, and a digital terrestrial television system, and put into practical use. Further, the OFDM technique is scheduled for use in a next-generation cellular phone system.
The optical OFDM communication system is a communication system applying the OFDM technique with light as a carrier. In the OFDM technique, a large number of subcarriers are employed as described above. Further, a multilevel modulation system such as 4-QAM, 8-PSK, or 16-QAM can be applied to a modulation system of each subcarrier. Therefore, one symbol time becomes very longer than an inverse of the bit rate. As a result, the transmission distance limited by the above-mentioned wavelength dispersion or polarization mode dispersion becomes sufficiently longer than a transmission distance (for example, 500 km in a domestic trunk system) assumed in the optical communication system, thereby making the above-mentioned dispersion compensating fiber unnecessary. As a result, there is a possibility that the optical communication system can be realized at the low costs. As a specific numerical example, let us consider a case in which the optical communication system having the bit rate of, for example, 10 Gbps is realized by the optical OFDM technique. When it is assumed that the number of sub-carriers is 10, and the modulation of each subcarrier is 4-QAM, one symbol speed is 500 MBaud. The transmission distance limited by the chromatic dispersion in this case becomes (10/0.5)2=400 times of a related-art on-off keying (OOK) system that conducts on-off modulation in the optical communication system of 10 Gbps, that is, 24000 km. Thus, the domestic trunk system having the transmission distance 500 km can be realized without using the expensive dispersion compensating fiber, and the low-cost optical communication system can be realized.
The optical OFDM communication system can be classified into two types according to a receiving scheme of an optical signal. One type is a direct detection receiving scheme, and the other type is a coherent receiving system. The present invention relates to an optical OFDM communication system using the direct detection receiving scheme.
The configuration diagram of this system is illustrated in FIG. 3. When data to be originally communicated is input to a transmitter 1 from an input terminal 9, the data is converted into a baseband OFDM signal in a transmission signal processor 100 within the transmitter 1. The converted signal is amplified by a driver amplifier 2, and carried on a light, which is a carrier, by an optical modulator 4 to generate an optical OFDM signal. The optical OFDM signal passes through an optical fiber 5, which is a transmission channel, and arrives at a receiver 6. The optical OFDM signal is received by direct detection by a photodiode 7, and converted into an electric signal. The electric signal is ideally the above-mentioned baseband OFDM signal, and the signal is amplified by a preamplifier 8, demodulated to data to be originally communicated, by a reception signal processor 200, and output from an output terminal 10.
A functional configuration diagram of the transmission signal processor 100 is illustrated in FIG. 5, and a functional configuration diagram of the reception signal processor 200 is illustrated in FIG. 6. The data to be communicated is first converted into 2N parallel data by a serial-parallel converter 110. In this example, N is the number of subcarriers for carrying data. If the modulation of the subcarriers is 4-QAM, 2N pieces of parallel data are obtained. For example, in a case of 16-QAM, 4N pieces of data are obtained. That is, serial data is converted into “(the number of bits per one symbol)×(the number of subcarriers)” pieces of parallel data. A subcarrier modulator 120 modulates N subcarriers with the parallel data. The modulated subcarriers are converted into time data by an inverse FFT unit 130, and converted into serial data by a parallel-serial converter 140. A cyclic prefix is inserted into the serial data by a cyclic prefix insertion unit 150, and the signal passes through a DA converter 160, and is sent to a driver amplifier as an analog signal.
In the reception signal processor 200, a reception electric signal amplified by the preamplifier is converted into a digital signal by an AD converter 210. The cyclic prefix is removed from the digital signal by a cyclic prefix removing unit 220, and the signal is converted into N parallel data by a serial-parallel converter 230. The parallel data is separated into N subcarrier signals in an FFT unit 240, and data carried by each subcarrier is demodulated by a subcarrier demodulator 250, and converted into serial data by a parallel-serial converter 260.
A spectrum of the optical OFDM signal that propagates through the optical fiber 5 is conducted by using a single sideband wave modulation system for the purpose of avoiding an influence of chromatic dispersion in the optical fiber. An optical spectrum of the optical OFDM signal in this case is illustrated in FIG. 8. Subcarrier signals are arrayed at a higher frequency side of the carrier of light (the subcarriers may be arrayed at a lower frequency side). The optical spectrum of the optical OFDM signal has plural subcarrier signals arrayed at regular intervals each of which is an inverse Δ of one symbol time Ts. A signal band B occupied by the optical OFDM signal is about N×Δ assuming that the number of subcarriers is N. When the signal is received by direct detection, beat signals between the adjacent subcarriers occur due to direct detection conducted by the photodiode 7, that is, photoelectric conversion. The beat signals interfere with the subcarrier signals to be originally received, to thereby distort a reception signal. As a result, a receiving sensitivity is deteriorated.
Up to now, this problem (hereinafter referred to as “sensitivity deterioration by inter-subcarrier interference (ICI)”) is solved by, for example, the following four solution techniques.
A first technique is a guard band system disclosed in Non-patent literature 1, for example. A schematic diagram of a spectrum of a baseband OFDM signal generated in this system, and a schematic diagram of a spectrum of a reception electric signal occurring when the baseband OFDM signal is received by direct detection are illustrated in FIGS. 10A and 10B, respectively. In this system, the subcarrier signals carrying the signal to be originally communicated are separated from direct current by a signal band B to provide a guard band. The signal is converted into an optical OFDM signal and transmitted, and directly detected. The inter-subcarrier interference (ICI) is generated between the direct current and the signal band B, and the interfered subcarriers are separated in frequency domain from the subcarriers carrying data to be originally communicated, and do not interfere with the latter.
A second technique is a guard band system illustrated in Non-patent literature 2. In this system, the guard band is provided to the spectrum of the base band OFDM signal as in the first solution. However, in this technique, a bias point of the optical modulator 4 is set to a zero point (transmittance null) of a so-called “transmission characteristics” where no lightwave carrier occurs. A certain frequency (for example, −fc) component of the baseband is used as a carrier, and the guard band is set for the signal band B of the carrier whereby the subcarriers carrying the signal are arrayed at the higher frequency side. A schematic diagram of the spectrum of a specific baseband OFDM signal, and a spectrum of an electric signal when the signal is optically transmitted, and directly received are illustrated in FIGS. 11A and 11B, respectively. A difference between this technique and the above first technique resides in that the spectrum of the baseband OFDM signal is shifted by −fc. Accordingly, the spectrums of the electric signals received by direct detection are identical with each other (refer to FIGS. 10B and 11B).
A third technique is disclosed likewise in Non-patent literature 2. In this solution, the guard bands according to the first and second techniques are arrayed between the subcarriers carrying the signal. Specific frequency layouts are illustrated in FIGS. 12A and 12B. In a spectrum of the baseband OFDM signal at the transmitter side illustrated in FIG. 12A, each interval between the adjacent subcarriers carrying the signal is spaced by 2×Δ. This signal is converted into the optical OFDM signal, and transmitted. An electric signal spectrum generated by receiving the signal by direct detection is illustrated in FIG. 12B. The inter-subcarrier interference (ICI) is generated between the adjacent subcarrier components carrying the signal, and no interference with the signal occurs.
A fourth technique is disclosed in Non-patent literature 3. A spectrum of the optical OFDM signal is illustrated in FIG. 8. In this technique, in a receiver, after normal signal processing is conducted to decode the subcarriers, a distortion component attributable to the inter-subcarrier interference is generated by signal processing with the use of the decoded data, and an influence of the inter-subcarrier interference is reduced by subtracting the distortion component from the received signal.    Non-patent literature 1: A. J. Lowery, L. Du, and J. Armstrong, “Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems”, OFC2006, postdeadline papers, PDP39, 2006    Non-patent literature 2: W. Peng, X. Wu, and V. R. Arbab, et al, “Experimental demonstration of a coherently modulated and directly directed optical OFDM systems using an RF-tone insertion”, OFC2008, OMU2, 2008    Non-patent literature 3: W. Peng, X. Wu, V. R. Arbab, et al, “Experimental demonstration of 340 km SSMF transmission using a virtual single sideband OFDM signal that employs carrier suppressed and iterative detection techniques”, OFC 2008, OMU1, 2008